CDMA (Code Division Multiple Access) is a spread spectrum communication technology, and it is used mainly for military communication at its initial stage. Because of its excellent performances, CDMA is widely used in the commercial communication area. Also, commercial CDMA cellular mobile communication systems have already been operated in the telecommunication networks.
In a CDMA system, the same carrier frequency can be shared by different users, and they are distinguished from each other by using different code words. Likewise, a user can use a plurality of code words to communicate simultaneously on a certain frequency. If these code words belong to different cells, a plurality of wireless links will communicate simultaneously. The state of a plurality of wireless links existing simultaneously between a mobile station and a base station is referred to as macro diversity. Macro diversity allows communication between a mobile station and a base station not to be interrupted during a switch between the same frequency. Meanwhile, because of utilization of diversity technology, a mobile station and a base station can obtain better link quality.
When a mobile station initializes a call and makes a request for accessing, a base station performs an admission control. An admission control is a rather complicated procedure which has to consider a variety of factors. In a CDMA system, one of the objectives of an admission control is to ensure an idle wireless resource for an expected SIR and bit rate. When a mobile station initializes a call in a new cell by a new calling or switching, an admission control is usually performed. Also, when a mobile station asks to dynamically reassign channels for reasons such as a change of service type during communication, an admission control is also performed. A CDMA system is a self-interfering system. While considering wireless resources, system interference affected by a user access is generally considered at the same time. When wireless resources of a certain cell are limited or the load thereof is too heavy, an admission of a mobile station may be failed. However, the failure of a cell does not necessarily mean that the other cells in the system do not possess resources. A mobile station may be able to move to a certain suitable cell, thereby accessing or communicating continuously. A procedure after an admission of an accessed cell or after a current cell being failure but a mobile station accessing to another cell under control of a base station, is referred to as direct retrying, also directional retrying.
In GSM, when a dedicated channel is assigned to a mobile station, and no dedicated channel resources are possessed by an access cell, direct retrying (or referred to as directional retrying) will be implemented. When accessing, a mobile station carries a measurement report of adjacent cells, which provides a foundation for selecting a target cell by direct retrying after access is failed. Patents, such as WO9856203 and U.S. Pat. No. 5,293,641, have discussed direct retrying in great extent. However, in some mobile communication systems or at some moments, direct retrying may not be supported by measurement reports. At this time, other methods may be used to select a target cell to improve a success rate of a user access, to decrease the number of frequency switch times and signaling interacting caused by sliding after a mobile station is accessed successfully, thereby optimizing system performance.